Scanner calibration is a well-established process directed to reducing the defects resulting from illumination and sensor array sensitivity non-uniformity. Typically, a scanner is calibrated to a white surface, often referred to as a calibration strip. In some scanners, such as in a typical flatbed scanner configuration, the effect of contaminant particles, such as dust and debris, on the calibration strip is insignificant. A moving scan-head will move across the calibration strip to read an area of the surface, for example, for a total of 256 lines. Scanned data are then averaged by column and used to adjust the system responses.
However, in some scanner configurations, such as a sheet fed automatic document feeder configuration, where neither the scan-head nor the calibration strip can be moved, only one position on the calibration strip typically can be read. Small particles such as dust and debris may contribute significantly since the scan line width is comparable to the pixel width. Any dust or debris on that line may be falsely considered as a sensitivity weakness, and the gain factor may be falsely exaggerated. Such errors will be shown as bright vertical streaks on the scanned images.
To prevent streaking caused by small particle contamination, scanners should compensate for the contaminant particle. One contamination compensation approach employs software detection of improbable sudden changes in system sensitivity to conclude the existence of small particles. Upon detecting the particles, the sensitivity values at these locations are discarded and replaced with surrounding valid pixels. Under this approach, the assumption is that the sensor sensitivity non-uniformity is usually minimal and can be set to a limit, e.g. 20%. Streaking may still occur even at these low sensitivity values.
As demand increases for improved scanner resolution and clarity, the need arises for improvements in components and methods relating to scanner calibration, particularly in identifying the location and dimensions of contaminant particles on a calibration strip and compensating for these contaminant particles during scanner calibration.